Recently, string instanton effects have been intensively
explored in moduli stabilization of flux induced compactifications
[1]}, [2]}, [3]} (and references therein) and string
phenomenology, especially for non-perturbative generation of
right-handed neutrino masses and \(\mu \)  term in intersecting
brane models [4]}, [5]}, [6]}, [7]}. These
non-perturbative effects come from nonzero global charges
\(Q_{a}=N_{a}\Xi \circ (\Pi _{a}-\Pi ^{^{\prime }}_{a})\)  carried by the
instanons, which lead to interesting charged matter couplings [8]}
(for recent reviews, see for example, [9]}, [10]}).
Setting the string scale at the order of \(10^{18}\)  GeV,
one finds \(m_{\nu }\)  and \(\mu \)  in acceptable ranges without any fine-tuning.
